A main function of modern computers is to
enable communications - sending electronic mail, news bulletins,
and documents across the office or around the world. After all,
a computer by itself is really nothing more than an overgrown programmable
calculator - a word processor with delusions of grandeur.
But with a modem or a network interface, computers can "speak"
and send information.

A good communications infrastructure
works both ways: not only does it let you get information
out
,
it also lets you get back
in
to your computer
when you're at home or out of town. If your computer is
equipped with a modem that answers incoming telephone calls, you
can dial up when you're sick or on vacation and read your
electronic mail, keep informed with your online news services, or
even work on a financial projection if the whim suddenly strikes
you. You can almost believe that you never left the office in the
first place!

But in the world of computer security,
good communications can be a double-edged sword. Communications
equipment can aid attackers and saboteurs while it enables you to
get information in and out easily. As with most areas of computer
security, the way to protect yourself is not to shun the technology,
but to embrace it carefully, making sure that it can't
be turned against you.

Modems
are devices that let computers transmit information over ordinary
telephone lines. The word explains how the device works: modem is
an acronym for "modulator/demodulator."
Modems translate a stream of information into a series of tones
(modulating) at one end of the telephone line, and translate the
tones back into the serial stream at the other end of the connection
(demodulating). Most modems are
bidirectional
- every
modem contains both a modulator and a demodulator, so a data transfer
can take place in both directions simultaneously.

Modems
have a flexibility that is unparalleled by other communications
technologies. Because modems work with standard telephone lines,
and use the public telephone network to route their conversations,
any computer that is equipped with a modem and a telephone line
can communicate with any other computer that has a modem and a telephone
line, anywhere in the world. Thus, even in this age of the Internet
and local area networks, modems are still the single most common
way that people access computers remotely. This trend is likely
to continue into at least the near future, especially with the continuing
popularity of specialized dial bulletin board systems (
BBSS
).

Early computer modems commonly operated at 110 or 300 baud,
transmitting information at a rate of 10 or 30 characters per second,
respectively. Today, in 1996, few computer modems are sold that
are not capable of 14,400 bits per second (bps), and modems that
zip along at 28,800 bps are increasingly popular. Some modems that
send data synchronously, with a precision clock, are capable of
rates in excess of 100,000 bps. Special modems on digital
ISDN
lines are also capable of speeds in excess of 100,000 bps. With
data compression included, and new technology constantly being offered,
we expect to see common modems with increasingly higher speeds (and
smaller physical sizes) as time goes on.

Baud
is named after the 19th-century French inventor, J. M. E. Baudot.
He invented a method of encoding letters and digits into bit patterns
for transmission. A 5-bit descendent of his code is still used in
today's
TELEX
systems.

5
to 12 bits are required to transmit a "standard"
character, depending on whether we make upper/lower case
available, transmit
check-bits
, and so on.
A multi-byte character code may require many times that for each
character. The standard
ISO
8859-1 character
set requires eight bits per character, and simple
ASCII
requires seven bits. Computer data transmitted over a serial line
usually consists of one
start bit
, seven
or eight
data bits
, one
parity
or
space bit
, and one
stop bit
.
The number of characters per second is thus usually equal to the
number of bits per second divided by 10.

The word "baud[1]"
refers to the number of audible tokens per second that are sent
over the telephone line. On 110- and 300-bits-per-second (bps) modems,
the baud rate equals the bps rate. On 1200-, 2400-, and higher-bps
modems, a variety of audible encoding techniques are used to cram
more information into each audible token.
TDD
phone devices for the deaf use a lower-speed modem than modern computers
usually do.

[1] The "baud" is not to be confused with the
"bawd," which is the rate at which juveniles transmit
risqué pictures over network connections.